This invention relates to methods for investigating subsurface earth formations. More particularly, this invention relates to methods for determining the permeability and other hydraulic properties of earth formations surrounding boreholes.
The determination of permeability and other hydraulic properties of formations surrounding boreholes is very useful in gauging the producibility of the formations, and in obtaining an overall understanding of the structure of the formations. For the reservoir engineer, permeability is generally considered a fundamental reservoir parameter, the determination of which is at least equal in importance with the determination of porosity, fluid saturations, and formation pressure. When obtainable, cores of the formation provide important data concerning permeability. However, cores are difficult, expensive, and time consuming to obtain, and even when performed only provide information about very small samples. Thus, in situ determinations of permeability which provide logs of horizontal and vertical permeabilities at a length scale greater than that provided by cores are highly desirable.
Existing techniques for making permeability determinations can be classified into indirect and direct methods. In indirect methods, permeability is determined from empirical correlations which attempt to express permeability in terms of other measured formation parameters, such as porosity, saturation, or mineralogy. A direct measurement technique involves actual measurement of fluid flow, pressure, etc. and determination of permeability from these measurements.
Different devices have been used for making direct measurements of permeability. For example, devices whose primary use has been for sampling formation fluids, have also been used with some success in estimating formation permeability. Formation testing devices which can take repeated samples are disclosed, for example, in U.S. Pat. Nos. 3,780,575 and 3,952,588. In these devices, a hydraulic pump provides pressure for the operation of various hydraulic systems in the device. Sample chambers are provided in the tool to take samples of formation fluid by withdrawing hydraulically operated pistons. Pressure transducers are provided to monitor pressure as the fluid is withdrawn, and pressure can be continuously recorded. So-called pre-test chambers are also typically provided and are operated to permit more reliable flow during the subsequent fluid withdrawal. Filters can also be provided to filter sand and other particulate matter, and pistons can be provided to clean the filters, such as when the tool is retracted.
One type of formation testing device includes an elongated body and a setting arm activated by setting pistons which are used to controllably urge the body of the device against a side of the borehole wall at a selected depth. The side of the device that is urged against the borehole wall includes a packer which surrounds a probe. As the setting arm extends, the probe is inserted against the formation, and the packer then sets the probe in position and forms a seal around the probe, whereupon the fluids can be withdrawn from the formation during pre-test and the actual test.
Existing formation sampling devices have been of limited usefulness in determining formation permeability for a number of reasons. In some instances, attempts have been made to use pressure measurements during fluid withdrawal as an indicator of permeability. If fluid is extracted at a fixed flow rate which is independent of permeability, as is typically done, in low permeability formations the pressure drop tends to be too large, and solution gas and/or water vapor forms and can make the results uninterpretable. On the other hand, at high permeabilities, the pressure drop tends to be too small and cannot be accurately measured.
In U.S. Pat. No. 2,747,401 there is disclosed a method and apparatus for determining hydraulic characteristics, including permeability, fluid pressure, and hydraulic anisotropy, of formations surrounding a borehole. A pressure gradient is obtained in the formations by inserting a probe through the borehole wall. Pressure differences between different points are then used to obtain indications of hydraulic characteristics of the formations. In an embodiment disclosed in the patent, a pair of spaced probes are inserted into the formation, and a pressure gradient is generated by inserting a fluid into the formation at one of the probes (a source probe) at a constant flow rate. The other probe (a measurement probe) is coupled to a pressure responsive device. Pressure is measured at the measurement probe before and after injection of the fluid at the source probe. The permeability of the formation is then obtained using a formula in which permeability is proportional to viscosity times flow rate divided by the change in pressure. The patent points out that the pressure gradient can also be obtained by extracting fluid from the formation and that measurements can be made in more than one direction; e.g., vertical and horizontal, to obtain indications of both vertical and horizontal hydraulic characteristics.
In improving upon the previous permeability tools, another method and apparatus for determining hydraulic properties of a formation is set forth in U.S. Pat. No. 4,742,459 to Lasseter, which is hereby incorporated by reference herein in its entirety. In the Lasseter patent, a logging device is provided having a source probe, a horizontal observation probe which is azimuthally displaced on the borehole wall with respect to the source probe position, and a vertical observation probe which is vertically displaced on the borehole wall with respect to the source probe position. The source probe is provided with means for withdrawing fluid at a substantially constant pressure, while the vertical and horizontal probes, as well as the source probe, are provided with means for measuring formation pressure response as a function of time. According to the method for determining permeability, a transient pressure change is established in the formation by withdrawing fluid from the formation at the source probe location. The formation pressure response is then measured at the vertical and horizontal probes. By selecting a trial permeability value, theoretical formation pressure responses can be derived as a function of time at the probe locations. The theoretical formation pressure responses are then compared with the actually measured pressure responses in an iterative manner, with the difference being used as feedback to modify the trial value, until the difference is negligible.
While the prior art patents, including the Lasseter patent, have had varying degrees of success in determining the hydraulic characteristics of the borehole, extremely accurate determinations over a wide range of permeabilities have not been obtainable utilizing the prior art methods. Typically, the difficulties encountered by the prior art techniques relate to the requirement of a measurement of flow rate, or the requirement that the formation fluids be withdrawn at a constant flow rate.